Device for metering digester side relief liquor



DEVICE FOR METERING DIGESTER SIDE RELIEF LIQUoR Filed on, 1ra, 1951 A. HELLAND 2 Sheets-Sheet 1 De@ l, 1964 A. HELLAND 3,159,527

DEVICE FoE METERENG DIGESTEE sIDE RELIEF LIQuoE Filed Oct. 16. 1961 2 Sheets-Sheet 2 SIDE RELIEF HEADER TO LOW-PRESSURE RELIEF HEAT EXCHANGER LEVEL CONTROL 25 FL 0W CONTRO L 9 Y A I/ F y To No.1 AccuMuLAToR .3l

United States Patent 3,159,527 DEWQE EUR METEEN@ DEESTER SEDE REMEF MQUR Aage Heiland, Kapusltasing, ntario, Canada, assigner to Spruce Fails Power and Paper Company, Limited, 0n-

tario, ntario, Canada, a corporation of Canada Fiied Get. 16, 196i, Ser. No. 145,260 1 Claim. (Ci. 162-29) This invention relates to an improved method of relieving digesters in pulp manufacturing processes. More particularly it relates to an improved method of metering digester side relief to assure uniform cooking of the mass of brous material being digested and to achieve a maximum yield of high quality pulp while recovering useful components from the spent cooking liquor.

The invention is peculiarly useful for pulping operations employing bisulfite liquors, but it also is adaptable for employment with any digestion liquor which contains a volatile component, such as sulfurous acid solutions, solutions of ammonia or nitrogen derivatives, and the like.

A major object of the present invention is to provide an improved method of metering digester side relief in a positive manner to assure accurate measurement of the amount of liquor removed and to achieve reproducibility and standardization of the pulp products obtained in each cook.

An equally important object is to control digester side relief to minimize potential variations in the amount of cooking liquor removed, thus assuring that the entire mass of iibrous material being digested is cooked completely in the liquid phase and not partially in the vapor phase.

Another object is to obtain maximum recovery of the useful gaseous, liquid, and heat components of the digestion liquor While the above objects are accomplished.

Other objects and advantages will become readily apparent lto persons skilled in the art upon examining the following description and drawings, as will Various modications, without departing from the inventive concepts defined in the appended claim.

In the drawings:

FIG. 1 is a schematic iiow diagram illustrating a typical embodiment of the invention.

FIG. 2 is a schematic diagram showing the side relief metering device in more detail.

The conventional method for preparing suliite pulp comprises cooking wood chips, or other sources of fibrous cellulose in a bisullite liquor containing a controlled excess of free sulfur dioxide.

In the conventional process the digester is illed with wood chips the cover is secured in place, and hot cooking acid pumped in from an acid accumulator where relatively concentrated acid liquor is stored at elevated pressure and temperature. When the acid pumping first starts, an air vent valve located near the top of the digester is opened, Venting to the atmosphere the free air entrained between the chips. When gas from the digestion liquor begins to appear in the vented air, the air vent valve is closed and the gas flows through a low pressure valve toa reclaimer Where it is absorbed by raw liquor owing to a low pressure accumulator. When the digester is full of acid the low pressure Valve is closed and a high pressure relief valve opened to a discharge line which returns the high pressure relief gases and liquor to a high pressure acid accumulator into which fresh acid is simultaneously being pumped.

VAfter the digester'is illed with acid, pumping is continued to increase hydrostatic pressure to about 30 p.s.i.

, At this stage the acid delivery pump is shut down `and the delivery valve closed. An acid circulating pump is then it by direct steam injection, or indirectly by circulation through an external heater. This step gradually raises pressure and temperature to the level desired for cooking.

As the temperature in the digester is raised, pressure increases because of the escape of sulfur dioxide from the liquor; and the volume of liquor increases as a result of expansion, and increased volume of condensate when direct steam is used. The pressure in the digester is regulated automatically by relieving sulfur dioxide and some acid through the high pressure relief valve. Maximum pressure in sulfite digesters is usually held at to 85 p.s.i., although some digesters have been built for higher pressures. To allow for expansion of the liquor in the digester, a certain amount of side relief is also taken od to maintain a gas space at the top of the digester. This side relief is taken only after maximum reaction temperatures have been reached, usually between the fourth or sixth hours. As explained hereinafter, this invention is concerned primarily with controlled metering of such side relief.

Upon completion of the conventional cook, pressure within the digester is usually blown down from maximum to between about 20 to 25 psi., after which the discharge valve at the bottom of the digester is opened and the cooked contents are blown into a blow pit.

As previously noted, the gas pressure in the digester is constantly relieved through a high pressure relief valve as the temperature rises. In modern mills this relief gas is collected in an accumulator system. Accumulators are usually spherical steel vessels lined with acid-proof brick and generally both a high-pressure and a low-pressure accumulator are employed. Raw acid from a storage tank is pumped through both accumulators in turn. Hot relief gas from the digesters and ythe side relief liquor from the early part of the cook are conducted into the high pressure accumulator to increase 'the percentage of free sulfur dioxide in the acid by counterflow absorption and to salvage some of the heat which otherwise would be lost. Low pressure relief circuits also return ashed oli gas to the low-pressure accumulator to enrich the fresh acid introduced therein.

The high-pressure accumulator is generally designed for a maximum pressure operation of 50 p.s.i. When this pressure is reached, the gas from the high-pressure accumulator passes into the low-pressure accumulator, which generally operates at about 20 p.s.i. Any over gas from the low-pressure accumulator is then conducted to the raw-acid storage tanks, where it is absorbed. Gas relieved during the blowdown of the digesters at the end of the cook goes into one or the other of the accumulators, depending on the blowing pressure. Thus, during the course of the digester cooking cycle, the concentration of free sulfur dioxide and the temperature of the acid in the high-pressure accumulator are raised to the level desired for cooking. The liquor from the high-pressure accumulator is then used in lling a fresh digester as previously described.

The present invention is directed particularly to an improved method for controlling the amount of side relief liquor removed from the digester during the side relief stage of the process.

The control and measurement of the amount of side relief has been a long-standing problem in the industry. Several methods of control or measurement have been tried, including gauge glasses, level recorders, and ow meters. All of these methods have failed for the same reasons; namely, the impossibility of obtaining an accurate measure of Volume from a mixture of hot liquid and gas under conditions of falling pressure. The gas which flashes olf under these conditions mixes in varying amount with the liquid in the metering device and destroys the measurement.

To overcome the above-mentioned difficulties, the process of this invention provides means for separating the hot side relief liquid and gas under reduced pressure, and then employs a high pressure pump to force the separated liquid through a fiow controller or meter to the high pressure acid accumulator while taking an accurate measurement of the volume of ow. The rate of flow of side relief liquor from the digester thus can be adjusted to remove the amount of liquor determined necessary for standardizing the cooking process and obtaining reproducible uniform quality for each cook. The process is especially applicable to a system which employs a plurality of digesters operated sequentially.

The old procedure for side relief operated on an unpredictable time-volume relationship. For example, in an 8-hour cooking schedule, a digester was normally put on side relief at about the fourth hour and relieved for one and one-quarter hours. In this system, side relief liquor was fed directly from the digester where the pressure was about 85 p.s.i. to the high pressure acid accumulator, where the pressure was 50 p.s.i., with the pressure differential providing the drive for what was assumed to be a uniform flow resulting in a uniform volume of relief for similar time periods. Thus for a relief period of one and one-quarter hours, it was assumed the volume of relief liquor would be the same for all digesters under the above conditions. This assumption was contrary to fact. It was found that the amount of side relief acid removed under the above conditions of pressure differential and time was never constant. In many instances, too much liquor was relieved and, during the period of cooking following relief, some of the material was cooked in the vapor phase rather than the desired liquid phase. In other instances, not enough liquor was removed, due to strainer plugging or like conditions, and additional unscheduled side relief was required.

This invention provides a means for accurately measuring and predetermining the volume of side relief acid to be removed in each cook, thus assuring reproducibility in the cooking process while recovering useful components from the relieved liquor.

The invention will be more readily understood by referring to the accompanying drawings and the following description.

As shown in FIG. l, the major units of a typical pulping operation usually comprise a series of high pressure digesters 10, a high pressure hot acid accumulator 14, a low pressure acid accumulator 26, hot gas reclaimers 22 and 18, a low pressure heat exchanger 27 and a raw acid storage tank 21. In addition to these major elements, the process of this invention requires a low pressure side relief separator tank 28.

While only one digester is shown, it will be understood that this is indicative of a battery of any desired number.

In the pulping process, digester 10 is packed with wood chips through top opening 5. A typical charge is about 30 to 32 cords of wood to obtain about 18 tons of pulp. After the digester 10 is filled, the cover 12 is secured in place, and vent valve 20 between digester 10 and vent header 16, and air vent valve 3 from vent header to atmosphere are opened. Acid pump 13 is now started to deliver hot cooking acid from accumulator 14 through acid valve 11 to digester 10. A typical amount of acid is about 26,000 gallons. About twenty to twenty-five minutes are required to fill the digester. In about five or six minutes, free air is completely expelled from the digester and when free gas begins discharging through air vent valve 3, it is closed and the venting continued through valve to reclaimer 18 where the SO2 is absorbed by raw acid being delivered to low pressure accumulator 26 through reclaimer 18. When the digester 10 is full of acid, vent valve 20 is closed and the acid pump 13 left running until pressure builds up to about 30 p.s.i. At this point, valve 11 is closed and acid pump i3 shut down. As soon as acid pump 13 is shut down, the circulating system is brought into operation by opening valve 4 and starting circulation pump 15. At this stage, temperature control 17 calls for steam and opens steam control valve 19 to deliver steam through injector or hydroheater 25. The high temperature steam, and condensate formed, cause the pressure in the digester to build up to the desired cooking pressure of about p.s.i. Temperature rises to the desired maximum cooking temperature of about l35l40 C. The digester pressure of 85 p.s.i. continues to be automatically controlled by relief of acid and gas through high pressure control valve 23. The acid and gas relieved by valve 23 flows through valve 6 and high pressure header 26 directly to high pressure acid accumulator 14.

At the end of about four hours of cooking at the desired pressure and temperature, side relief valve 7 is opened permitting relieved cooking acid and gas to flow via common header 29, which services all digesters, to side relief separator Z3. lt will be understood that the intake of the side relief line is provided with the usual screening means to prevent passage of liber through the relief line. When side relief valve 7 is opened, high pressure relief through high pressure control valve 23 will discontinue, due to the drop in pressure, as side relief acid flows to the separator 2S Where the gas and liquid phases separate under a predetermined low pressure maintained therein by pressure control valve 33. In this example, the low pressure valve 38 was set at about 55 p.s.i., although other suitable pressures may be employed. Thus, in this example, the pressure differential between digester 10 and separator tank 28 is about 30 p.s.i. at the time side relief is started.

The degassed side relief liquor accumulating 4in separator tank 2S is pumped out by pump 31 at a predetermined rate of flow set on flow control 34 which operates flew control valve 33. As the liquor is removed, the volume of liquor entering the separator 28 is controlled by level control valve 39. The purpose of level control 39 is to assure that as a predetermined metered amount of the separated liquid is removed from the separator tank 2S, sufiicient high pressure relief liquor is admitted from the digester to maintain the level of degassed liquid in the separator tank substantially constant. If a substantially constant level of degassed liquid is not maintained in the separator tank, the amount of side relief liquor drawn from the digester would not be measurable, and the purpose of metering flow of degassed liquor from the separator tank would be defeated. It is noted that the total volume of ow entering the separator tank in the form of gas-containing liquor is greater than the volume of separated gas-free liquid being removed, since the entering flow contains gases while the exiting flow is substantially degassed. However, the volume of the liquid component itself which is allowed to enter the eparator is substantially equal to the amount withdrawn therefrom because the level of separated degassed liquor is maintained substantially constant. The degassed side relief liquor removed from separator 28 is delivered through an acid line to high pressure accumulator 14.

The rate of flow from the separator tank is determined by the spacing of the digesters and is set to maintain a continuous controlled ow without overlapping. For example, if 3,000 gallons is the desired volume to remove from separator tank 2?, and the digester spacing is one hour, then the rate of controlled ow is ,Any desired rate of flow to fit particular digester spacing may be employed. As soon as the desired amount of side relief is removed from one digester, the next digester is in line to go on side relief in the above described manner, thus providing a continuous flow of side relief liquor to the high pressure accumulator 14. This continuously controlled rate of flow of side relief liquor to accumulator 14 assures more uniformity in the new cooking acid being prepared.

Pressure in side relief separator tank 28 is controlled by pressure'control valve 38, which in this case is set at 55 p.s.i., but which may be adjusted as desired. At the beginning of the flow of side relief acid from digester 10, the side relief header pressure will be about 85 p.s.i., gradually dropping as side relief continues. When the next digester in line is put on side relief, the pressure will suddenly rise again to about 85 p.si. The level control 39 maintains a steady liow throughout these Variations. As the pressure control 38 maintains an even pressure in the separator tank 28, the amount of gas relieved will Vary only slightly.V

If, because of a reblow or some other unforseen delay, a digester is out of step or not in proper sequence, it will be relieved through valve 8 until it lits back into the schedule. l

This ow control system insures that a predetermined amount of side relief is removed from each digester in turn, to provide uniform cooking and standardization of the pulp products obtained.

To complete the process after side relief of a digester is complete, the steam supply to that digester is shut off. Valve 6 is closed and valve 9 to low pressure relief headers 35 is opened, and pressure reduced from 40 p.s.i. to a final blow olf pressure of between about 20 to `25 p.s.i.

A record of ow may be continually provided at a suitable control panel. Separator tank level and pressure may be similarly recorded.

Adjustments in ow, pressure, and level can be made to suit conditions as found necessary from iield practice, and .to suit different digester spacing when necessary.

While preferred apparatus and methods of operation have been described, it is understood that these are given merely to exemplify the underlying principles involved. Obviously, modifications in structure and iioW will occur to those skilled in the art and such modifications are intended to be comprehended by the present disclosure.

The invention is not intended to be restricted to the apparatus or processes described, except as such limitations are clearly imposed by the appended claim.

What is claimed is:

A device for metering the amount of digester side relief liquor drawn from a high pressure pulp digester during the pulping process which comprises, a low pressure separator tank, supply means for continuously supplying high pressure digester relief liquor to said separator tank, pressure control means for controlling the pressure in said tank at a predetermined value substantially below the initial pressure of said relief liquor whereby said liquor is separated into gaseous and liquid components, said pressure control means being reactive to said gaseous component, level control means for maintaining constant the level of said liquid component in said tank by controlling the supply of high pressure relief liquor admitted thereto, and metering means for withdrawing said sepavrated liquid component at a` constant rate from said tank while said liquor is continuously fed thereto whereby a metered amount of relief liquor may be withdrawn from said digester.

References Cited in the file of this patent UNlTED STATES PATENTS 

